As known, numeric control milling machines or boring machines, traditionally called “gantry” type, essentially comprise a long horizontal main supporting crossmember with a highly rigid structure, which extends horizontally and perpendicularly to the longitudinal axis of the machine, at a predetermined height from the ground, and has two axial ends structured so as to rest stably and in an axially sliding manner on two horizontal rectilinear guides which extend parallel to the longitudinal axis of the machine, on the top of two side walls or buttresses raised from the basement; a movable slide which is fixed protrudingly and in an axially sliding manner on a series of rectilinear guides which extend along the side of the main supporting crossmember parallel to the longitudinal axis of said crossmember, so as to move along the crossmember in a horizontal direction locally parallel to the longitudinal axis of the crossmember; a movable vertical tower which is fixed onto the movable slide in a vertical position, also with the possibility of moving with respect to the slide in a vertical direction, so as to vary the distance from the ground beneath; lastly, a tool-holder head which is fixed to the lower end of the movable vertical tower, usually with the possibility of rotating around a vertical axis and/or around a horizontal axis, so as to reach any point of the article held on the basement beneath the main supporting crossmember.
Unfortunately, “gantry” type numeric control milling machines cannot perform mechanical processing with removal of material, which requires precision exceeding one hundredth of a millimeter, unless they have a main supporting crossmember with a largely over-sixed structure with respect to traditional uses, with the considerable increase in costs this involves.
On this type of numeric control machine, in fact, the slide and the vertical tower are fixed protrudingly onto the side of the main supporting crossmember, so the weight force associated with these two components produces, on the vertical plane, a mechanical moment which tends to tip over the movable vertical tower, tearing the slide from the side of the main supporting crossmember. This mechanical moment obviously discharges onto the body of the main supporting crossmember simultaneously with the normal mechanical bending stress due to the weight of the two components, and tends to twist the body of the crossmember in a variable manner as a function of the momentary position of the slide along the main supporting crossmember and the weight of the tool-holder head fixed onto the lower end of the vertical tower.
On the “gantry” type milling machines currently on sale, torsional and bending deformations of the main supporting crossmember are maintained within reasonable limits by means of appropriate over-sizing of the structure of said main supporting crossmember. This solution obviously heavily influences the total production costs of the machine.
Unfortunately, however, when the length of the horizontal main supporting crossmember exceeds 4-6 meters and the total weight of the slide and the vertical tower exceeds 2000 Kg, limitation of torsional deformations of the main supporting crossmember becomes technically and economically prohibitive, so the torsional and bending deformations of the crossmember start to negatively influence the degree of precision of positioning of the tool fixed onto the tool-holder head. This is why large “gantry” type milling and boring machines cannot guarantee, at reasonable costs, the same degree of precision offered by milling or boring machines of smaller dimensions.